Conventional steam boilers have a number of economic shortcomings and cause many operating problems in combined cycle applications, particularly those of small capacity using gas turbines in the 4000-25000 Kw range. Specifically:
Start-up and shut down procedures are complex. Attended operation is mandatory, and as many as seven hours may be needed to carry out the procedure.
A complex chemical feedwater treatment system is required. Such systems depend on operator vigilance for acceptable operation. The maintenance of chemical supplies and high make-up water requirements are added, related operating burdens.
Large numbers of hydro/pneumatic/mechanical devices are employed in the control systems of conventional boilers. These are subject to malfunction which makes maintenance costs high and reduces boiler availability.
Heat recovery is low because conventional boilers only produce steam at a single pressure. Consequently, even at high expense, only a moderate improvement in station heat rate may be obtained.
Tube, valve stem, joint and other leaks are common in conventional boilers. These are difficult and time consuming--and consequently expensive--to repair or plug. Excessive feedwater consumption and maintenance costs and deterioration in performance result.
Conventional boilers are also subject to freezing and related damage during installation and checkout.